Do preventive maintnence before mods. I was just reading a thread on werkz about some kid who had to sell his M3 because it "cost him too much to keep running". Turns out he put a supercharger on it... lol.
When doing preventive maintnence you can normally upgrade your car too. On most e36s the suspension is old and tired -- great time to go for shocks/springs or coilovers. Many people forget about that and just want power.